Blasthole drills are large earth drilling machines typically used in mining operations to drill holes for explosives. A conventional blasthole drill comprises a frame supported by crawlers for movement over the ground, and a mast supported by the frame for movement between a substantially vertical position and a number of angled or non-vertical positions. A rotary head moves along the mast. The rotary head engages the upper end of a drill pipe for rotating the drill pipe and driving the drill pipe into the ground. When drilling a blasthole that is deeper than the height of the mast, more than one section of drill pipe must be used. After the first section of drill pipe is driven into the ground, the rotary head moves back to the top of the mast and another section of drill pipe is connected to the top of the first section. The rotary head then drives the second drill pipe section into the ground. It is not unusual to use multiple sections of drill pipe. Such a deep blasthole is commonly referred to as a "multi-pass" blasthole. The various sections of drill pipe are usually supported relative to the mast and moved into position at the appropriate time by a device known as a pipe rack. One such pipe rack is disclosed in U.S. Pat. No. 5,622,232, which is incorporated herein by reference.
When drilling a multi-pass blasthole, a considerable amount of time is spent in adding and removing individual drill pipe sections from the overall drill pipe string. In the art of earth drilling machines, the process of separating drill pipe sections has long been one in which improvements have been sought. The problem of disconnecting or unthreading tightly connected joints between drill pipe sections has resulted in numerous devices for clamping and holding one drill pipe section while rotating the other drill pipe section.
Typically, an attempt is made to break the joint between two drill pipe sections by rotating the upper drill pipe section with the rotary head while the lower drill pipe section is held by the drill pipe holding wrenches. More often than not, the joint between the drill pipe sections is stronger or tighter than the joint between the rotary head and the upper drill pipe section. As a result, when using the rotary head to try and loosen the joint between two drill pipe sections, the joint between the rotary head and the upper drill pipe section usually breaks instead. Thus, if the joint between the drill pipe sections cannot be broken loose with the rotary head, a breakout wrench is commonly used to rotate the upper drill pipe section while a deck wrench holds the lower drill pipe section against rotation. An automatic breakout wrench is disclosed in U.S. Pat. No. 5,653,297, which is incorporated herein by reference. The automatic breakout wrench turns the upper drill pipe section while the lower drill pipe section is held by the deck wrench. This type of change-out procedure usually adds undesirable downtime to an overall blasthole drilling operation.
Conventional deck wrenches generally include a pair of opposed drill pipe holding members which are disposed on a drill deck and which are aligned with each other on opposite sides of an opening in the drill deck through which the drill pipe extends. Each drill pipe holding member is slideably mounted in a respective housing. Each section of the drill pipe generally includes a pair of recesses on opposite sides thereof. When it is desirable to unthread an upper drill pipe section from a lower drill pipe section, each drill pipe holding member engages a respective recess in the lower drill pipe section. The mating action of the members and recesses holds the lower drill pipe section against rotation in a single direction so that the upper drill pipe section can be unthreaded or disconnected from the lower drill pipe section.